16.10: UV–Vis Spectroscopy: Woodward–Fieser Rules
UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given structure by adding the contributions from various substituents to a base wavelength, which remains fixed for a given class of dienes or trienes. The applicability of Woodward–Fieser rules can be extended to α,β-unsaturated carbonyl compounds.
Woodward–Fieser Rules for Dienes
- Base value:
- Acyclic diene = 214 nm
- Homoannular diene = 214 nm
- Heteroannular diene = 253 nm
- Substituent contributions:
- Double bond extended conjugation = 30 nm
- Ring residue = 5 nm
- Exocyclic double bond = 5 nm
- Substituent groups:
- Alkyl group = 5 nm
- –OR = 6 nm
- –Cl, –Br = 5 nm
- –NR2 = 60 nm
Figure 1.
Calculation for Figure 1:
Base value (Heteroannular diene) = 214 nm
Three ring residues = 3 (5 nm) = 15 nm
One exocyclic double bond = 5 nm
One –OR group = 6 nm
λmax (calculated) = 240 nm
λmax (observed) = 241 nm
Woodward–Fieser Rules for α,β-Unsaturated Carbonyl Compounds
- Base value:
- Acyclic or six-membered cyclic enones = 215 nm
- Five-membered cyclic enones = 202 nm
- Substituent contributions:
- Double bond extended conjugation = 30 nm
- Exocyclic double bond = 5 nm
- Ring residue: α = 10 nm; β = 12 nm
- Substituent groups:
- Alkyl: α = 10 nm ; β = 12 nm
- –OCH3: α = 35 nm ; β = 30 nm
- –Cl: α = 15 nm ; β = 12 nm
- –Br: α = 25 nm ; β = 30 nm
- –NR2: β = 95 nm
Figure 2.
Calculation for Figure 2:
Base value (Acyclic enone) = 215 nm
One α –CH3 = 10 nm
Two β –CH3= 2 (12 nm) = 24 nm
λmax (calculated) = 249 nm
λmax (observed) = 249 nm
